Flywheel system with synchronous reluctance and permanent magnet generators

ABSTRACT

The present invention provides a flywheel system with a variable speed synchronous reluctance motor-generator and a variable speed permanent magnet generator for providing backup power. The flywheel system incorporates rotating elements supported by electromagnetic bearings. Electric power provided by the backup generator maintains electromagnetic bearing operation during that portion of the coast down period when shaft speed falls below the minimum required for operation of the synchronous reluctance motor-generator.

This application is a continuation of U.S. application Ser. No.11/251,394 filed Oct. 14, 2005, which claims priority from U.S.Divisional application Ser. No. 10/863,868 filed Jun. 7, 2004 now U.S.Pat. No. 7,109,622, which claims priority from U.S. ProvisionalApplication 60/476,226 filed Jun. 6, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the electromechanical arts and energystorage systems. In particular, the present invention relates toflywheel systems used for energy storage and conversion.

2. Description of Related Art

Flywheel energy storage systems have provided a mechanical energystorage solution for hundreds of years as evidenced by the potter'swheel. Such systems differ in many respects from modern-day flywheelenergy storage solutions. More recent design imperatives including highpower density and electric power outputs have led to lightweight,high-speed flywheels operating in evacuated chambers and driving asimilarly high-speed electric generator.

A typical application for today's flywheel is to provide electric powerto an electric network for a brief period of time, as might be neededwhen an electric power outage occurs. Such applications require that theflywheel operate in a stand-by mode, fully charged and ready to convertits mechanical energy into electrical power to support the electricalnetwork when network supply voltage droops.

To the extent that a protracted power outage occurs and the flywheel'susable electric output is depleted by the external electric network, theflywheel's internal electrical loads may be deprived of the electricpower required to complete a normal flywheel shutdown. Critical loadsinternal to the flywheel system may include electric and electroniccontrols.

Supplying electric loads internal to the flywheel system during coastdown presents a particular problem when the flywheel's electricgenerator has a minimum operating speed as is typical of inductivegenerators. Here, another source of electric power will be needed duringsome portion of the coast down period.

SUMMARY OF THE INVENTION

Now, in accordance with the invention, there has been found a flywheelsystem that provides electric power to critical loads during coast downdespite the absence of an external power source. A flywheel masssupported by electromagnetic bearings is rotatably coupled to amotor-generator for exchanging mechanical power with the motorgenerator. Further, the flywheel mass is rotatably coupled to a backupgenerator for converting mechanical energy from the flywheel mass intoelectrical power for providing electrical power to the electromagneticbearings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings that illustrate the present invention and, together with thedescription, explain the principles of the invention enabling a personskilled in the relevant art to make and use the invention.

FIG. 1 is a diagram showing modules included in the flywheel backuppower supply system constructed in accordance with the presentinvention.

FIG. 2 is a diagram showing elements included within the powerelectronics module of the flywheel backup power supply system of FIG. 1.

FIG. 3 is a diagram showing elements included in the flywheel module ofthe flywheel backup power supply system of FIG. 1.

FIG. 4 is a diagram showing regimes included in the operation of theflywheel backup power supply system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the flywheel system 100 of the present invention. Itincludes the flywheel module 102, power electronics module 104, andelectrical network 106. In the flywheel module, a first rotatablecoupling 120 interconnects the flywheel mass 112 with themotor-generator 116 and a second rotatable coupling 118 interconnectsthe flywheel mass with the backup generator 110. At least oneelectromagnetic bearing 114 provides rotatable support for the flywheelmass.

The power electronics module 104 is interconnected to sources andconsumers of electric power including the backup generator 110, themotor-generator 116, and the electrical network 106, and at least oneelectromagnetic bearing 114.

A first electrical circuit 122 conducts electric power unidirectionallyas shown by flow arrow 128 from the backup generator 110 to the powerelectronics module 104. A second electrical circuit 124 conductselectric power unidirectionally as shown by flow arrow 130 from thepower electronics module to at least one electromagnetic bearing 114. Athird electrical circuit 126 conducts electric power bi-directionally asshown by the opposed flow arrows 132, 134 between the motor-generator116 and the power electronics module. A fourth electrical circuit 108conducts electric power bi-directionally as shown by the opposed flowarrows 136, 138 between the power electronics module and the electricalnetwork 106.

Flywheel system 100 charging includes absorption and storage ofmechanical energy by increasing the rotational speed and hence kineticenergy of rotating elements within the flywheel module 102 including theflywheel mass 112. Flywheel system charging takes place when theelectrical network 106 supplies electric power as shown by flow arrow138 and the motor-generator 116 consumes electric power as indicated byflow arrow 132 while functioning as an electric motor.

Flywheel system 100 discharging includes releasing mechanical energy bydecreasing the rotational speed and hence kinetic energy of rotatingelements within the flywheel module 102 including flywheel mass 112.Flywheel discharging takes place when the electrical network 106consumes electrical power as shown by flow arrow 136 that is supplied bythe motor-generator 116 as shown by flow arrow 134 while themotor-generator functions as an electric generator.

FIG. 2 shows a first embodiment of the flywheel system includingselected elements of the power electronics module 200. The powerelectronics module 104 includes three electric power converters. Thefirst converter 202 is a uni-directional AC-to-DC converter, the secondconverter is a bi-directional AC-to-DC converter 204, and the thirdconverter is a bi-directional DC-to-DC converter 206. The powerelectronics module also includes an internal DC bus 222, an external DCbus 108, and a fifth circuit 224.

As mentioned above, the first, second, and third circuits interconnectthe power electronics module 104 and the flywheel module 102. Whatfollows is a description of selected sources and users of electric powerflowing in these circuits.

The first circuit 122 interconnects a backup generator AC output 240 asindicated by flow arrow 208 to a first converter AC input 264. The fifthcircuit 224 connects a first converter DC output 242 to an external DCbus tap 244 on the external DC bus 108. Electric power users includingthe electrical network 106 and the third converter 206 thereby receivebackup power from their respective interconnections 246, 248 with theexternal DC bus.

The second circuit 124 interconnects an electromagnetic bearing electricpower input 266 to an internal DC bus tap 268 on the internal DC bus222. Electric power flows from the internal DC bus to theelectromagnetic bearing(s) 114 as shown by flow arrow 130. The internalDC bus may receive electric power from the motor-generator 116, theelectric network 106 or the backup generator 110. When themotor-generator is providing electric power, the motor-generatorelectrical connection 256 is an AC output interconnected to a secondconverter AC connection 254 by the third circuit 126. Power flows from asecond converter DC connection 252 as indicated by flow arrow 134 to theinternal DC bus. When the electric network is providing electric power,an electric network connection 246 is a DC output interconnected to athird converter external DC connection 248 by external DC bus 108. Powerflows from a third converter internal DC connection 250 as indicated byflow arrow 214 to the internal DC bus. When the backup generator isproviding electric power, power flows as described above from the backupgenerator to the external DC bus and thereafter to the internal DC bus.

Turning now to electric power flows associated with charging anddischarging the flywheel system 100, the motor-generator 116 mayfunction either as an electric motor or as an electric generator. Duringcharging the motor-generator functions as an electric motor. Duringdischarging, the motor-generator functions as an electric generator.

During charging, the electric network 106 provides DC power to the thirdconverter 206 via external DC bus 108 as indicated by flow arrow 138.The converter adjusts the voltage to a level suitable forinterconnection with the internal DC bus 222 and transfers electricpower as indicated by flow arrow 214 to the internal DC bus. The secondconverter 204 takes power from the internal DC bus, synthesizes an ACoutput indicated by flow arrow 132, and transfers power to themotor-generator via third circuit 126. The AC output is suitable forpowering the motor-generator 116 for accelerating the flywheel 360 (seeFIG. 3).

During discharging, the second converter 204 receives electric powerfrom the motor-generator 116 via third circuit 126 as indicated by flowarrow 134. The converter adjusts the voltage to a level suitable forinterconnection with the internal DC bus 222 and transfers electricpower to the internal DC bus. The third converter 206 takes power fromthe internal DC bus and adjusts the voltage as required forinterconnection with the external DC bus 108. Flow arrows 216 and 136indicate transfer of electric power from the second converter to theelectrical network via the external DC bus.

It should be noted that although the electrical network 106 isinterconnected to the external DC bus 108, a person of ordinary skill inthe art will recognize that the electrical network may includeelectrical sources and loads having electrical characteristics thatdiffer from those of the external DC bus. Auxiliary electric powerconverters 230 provide for interconnecting such sources and loads to theextent they are present in the network.

It should also be noted that while output 242 of first converter 202 maybe processed by third converter 206 as shown in FIG. 2, in a secondembodiment, a fourth unidirectional DC-to-DC electric power converter(not shown) might be used to interconnect first converter output 242with the electromagnetic bearings 114. In this embodiment, the fourthconverter adjusts the voltage level at first converter output 242 toaccommodate the requirements of the electromagnetic bearings.

FIG. 3 shows selected flywheel module elements 300. Rotating elementsinclude the flywheel shaft 346 and the flywheel mass 112. Stationeryelements include the flywheel housing 358, first and secondelectromagnets 306, 318, and first and second electric stators 308, 342.The flywheel 360 includes the flywheel mass 112 and the flywheel shaft346. The flywheel shaft includes first and second sections 310, 314. Theflywheel shaft shares a common axis of rotation 322 with and is attachedto the flywheel mass.

The flywheel 360 has integrated features including antifriction bearings354, 356 and electromagnetic bearings 324, 328, a backup AC generator110, and a synchronous reluctance AC motor-generator 116. The sectionsthat follow provide details relating to these features.

Antifriction bearings 354, 356 provide rotatable support to the flywheel360 at low flywheel speeds. The flywheel utilizes first and secondtouchdown bearing shafts 302, 320 mated with respective first and secondantifriction bearings 354, 356 for rotatable support. The antifrictionbearings support both radial and thrust loads. The touchdown bearingshafts extend outwardly from respective opposing ends of the flywheelshaft and share a common axis of rotation 322 with the flywheel shaft346. The first and second antifriction bearings 354, 356 are fixed torespective first and second flywheel housing parts 362, 364.

Electromagnetic bearing(s) 114 provide rotatable support to the flywheel360 at higher flywheel speeds when the flywheel is no longer supportedby the antifriction bearings 354, 356, but now relies on at least oneelectromagnetic bearing for support. Here, first and secondelectromagnetic bearings 324, 328 are shown. The first electromagneticbearing 324 is proximate to the first shaft section 310 and includes anelectromagnet 306 attached to the first flywheel housing part 362 and anadjacent ferromagnetic portion 304 that is integral with the flywheelshaft 346. The second electromagnetic bearing 328 is proximate to thesecond shaft section 314 and includes an electromagnet 318 attached tothe second flywheel housing part 364 and an adjacent ferromagneticportion 316 that is integral with the flywheel shaft 346.

Each of the ferromagnetic portions of the shaft 304, 316 includes arespective plurality of thin ferromagnetic laminates 332, 336 havingelectrical insulation interposed between adjacent laminates. Theselaminated ferromagnetic structures increase the effectiveness of theelectromagnetic bearings by reducing eddy current losses. In particular,eddy currents induced in the ferromagnetic portions by theelectromagnets result in I² R heating losses. The thin ferromagneticlaminates reduce the magnetic flux in (results in smaller inducedvoltage) and the conductivity of (smaller conductive cross-section) eachferromagnetic laminate. The result is a reduction in eddy current lossesby a factor of approximately 1/n² where n is the number of lamella in aferromagnetic portion.

The backup generator 110 is a variable speed permanent magnet ACmachine. It includes a first electrical stator 308 adjacent to aflywheel shaft permanent magnet portion 330. The flywheel shaftpermanent magnet portion is in the first flywheel shaft section 310 andincludes a permanent magnet 348 integral with the flywheel shaft 346.

Since a permanent magnet generator is self-exciting, the backupgenerator generates electric power as long as the flywheel 360 isrotating even if no external source of electric power is available. Thebackup generator therefore provides electric power to theelectromagnetic bearings 324, 328 when operation of the electromagneticbearing(s) is desirable and when no other electric power source isavailable to operate the electromagnetic bearing(s). As a person ofordinary skill in the art will recognize, the power produced by thebackup generator may be used to power electric loads internal orexternal to the flywheel system 100.

The motor-generator 116 is any inductive AC machine known to ordinarypersons of skill in the art such as a wound-rotor or a reluctance typemachine. The motor-generator includes a second electrical stator 342 anda rotor 344.

In an embodiment, the motor-generator 116 is a variable speedsynchronous reluctance AC machine. Here, the motor-generator includes asecond electrical stator 342 adjacent to a flywheel shaft reluctorportion 344. The reluctor portion is in the second flywheel shaftsection 314 and includes a plurality of ferromagnetic reluctor poles 352integral with the flywheel shaft 346.

While functioning as an electric motor, the motor-generator 116transfers torque 332 to the flywheel shaft 346 increasing the rotationalspeed of the flywheel 360. While functioning as an electric generator,the motor-generator transfers torque from 366 the flywheel shaftreducing the rotational speed of the flywheel.

Since the motor-generator is not self-exciting, it produces electricpower only when induced electric currents magnetize the rotatingreluctor poles 352. An externally excited stator 342 that ismagnetically coupled with the reluctor portion induces such currents.Therefore, the motor-generator 116 cannot generate electric power unlessthere is a source of electric power external to the motor-generator. Thepower electronics 104 may provide the excitation power; however, whenthe flywheel 360 speed falls below a minimum value, useful generation ofelectric power by the motor-generator ends.

FIG. 4 is a graph 400 that illustrates the charging, charged, anddischarging cycle of the flywheel system 100. The vertical axis 402represents rotational speed of the flywheel mass 112 in revolutions perminute (RPM). The horizontal axis 404 represents time.

Starting from a stand-still and during pre-liftoff 406, flywheel systemcharging begins when the motor-generator 116 functions as a motor,applying an accelerating torque 332 to the flywheel shaft 346 aselectrical power is converted to mechanical motion. As the flywheel 360speed increases, the electromagnetic bearing(s) 324, 328 operate duringspeed range S1 to substantially disengage the touchdown bearing shafts302, 320 from the antifriction bearings 354, 356; this is termed“liftoff” 408.

During the post-liftoff period 410, the flywheel 360 is accelerated tothe maximum speed range S4. Upon reaching speed range S4, the flywheelis fully charged 412. Prior to discharging, the motor-generator cycleson and off as required to maintain flywheel speed within speed range S4.This cycling is required to recover speed decay resulting from frictionand other losses in the system.

While charging 424 and cyclically while charged 412, electrical powerfrom the electrical network 106 is conducted in the direction of flowarrow 214 via external DC bus 108, the third converter 206, the internalDC bus 222, the second converter 204, and the third circuit 126 to themotor-generator 116. Electric power supplied to the internal DC bus bythe electrical network also powers the electromagnetic bearing(s) 114via second electrical circuit 124 as indicated by flow arrow 130.

The discharging period 426 begins when the motor-generator 116 functionsas a generator, applying a retarding torque 366 to the flywheel shaft346 and converting the energy of mechanical motion into electric power.During this process, the rotational speed of the flywheel 360 isreduced. As the speed decreases from speed range S4 to speed S3, themotor-generator generates electric power. Note that similar flywheeldischarging occurs when the electrical network's external power source232 is interrupted: In this case, the power flow to the electricalnetwork 106 indicated by flow arrow 258 stops and the electrical networkbecomes dependent on the flywheel system for delivery of electric powervia external DC bus 108 as indicated by flow arrow 136.

During the initial discharging period 414, electrical power from themotor-generator 116 is conducted in the direction of flow arrow 134 viathe third circuit 126, the second converter 204, the internal DC bus222, the third converter 206, and the external DC bus 108 to theelectrical network 106 as indicated by flow arrow 136. Electrical powersupplied to the internal DC bus by the motor-generator also powers theelectromagnetic bearings 324, 328 via the second circuit 124 asindicated by flow arrow 130.

When the flywheel 360 reaches the minimum motor-generator speed S3, thesynchronous reluctance (inductive) motor-generator 116 is no longer ableto provide enough electric power to operate the electromagneticbearing(s) 324, 328. During the subsequent backup power speed regime416, the backup generator 110 provides sufficient electric power tooperate the electromagnetic bearings. The backup generator also provideselectric power for other electrical loads that may be necessary to thesafe shut-down of the flywheel. As one who is skilled in the art willrecognize, backup generator power is available via external DC tap 244and internal DC tap 268 for powering critical loads whether they beinternal or external to the flywheel system 100.

When touchdown 418 occurs in speed range S2, the electromagneticbearing(s) are no longer needed and the antifriction bearing shafts 302,320 are once again supported by respective antifriction bearings 354,356.

During the backup power speed regime 416, electrical power from thebackup generator 110 flows as indicated by flow arrow 208 via the firstconverter, the fifth circuit 224, the external DC bus 108, the thirdconverter 206, the internal DC bus 222, and the sixth circuit 124 to theelectromagnetic bearings 114 as indicated by the flow arrow 130. Here,the third converter is included in the power flow path to accommodatethe backup generator's variable voltage output that rises and falls withthe speed of the flywheel 360.

Post-touchdown 420 begins when the speed of the flywheel 360 falls belowspeed range S2. This regime is the final portion of the dischargingprocess 426. If a source of external power is not available, theflywheel will come to rest.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be understood by those skilledin the art that various changes in form and details can be made thereinwithout departing from the spirit and scope of the invention as definedin the appended claims. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. An flywheel system comprising: a rotatable assembly; the rotatableassembly having a single axis of rotation and including a flywheel mass,a motor-generator rotor and a permanent magnet; a fixed assembly; thefixed assembly including a first electrical stator adjacent to thepermanent magnet; a containment enveloping the rotating assembly; and,an electrical load critical to safe flywheel system operationelectrically coupled to the first electrical stator for receivingelectric power from the first electrical stator.
 2. The flywheel systemof claim 1 further comprising a second electrical stator adjacent to themotor-generator rotor.
 3. The flywheel system of claim 2 wherein themotor-generator rotor includes a plurality of ferromagnetic reluctorpoles.
 4. The flywheel system of claim 3 wherein a motor-generatorincorporating the second electrical stator and the motor-generator rotoris a variable speed synchronous reluctance motor generator.